I don't buy your argument. If opponents will be cruisers with 6" armament first place, why build a 18kts super-CA at all? A standard CA of about 12,000 tons can carry enough armour to be save against 15cm guns at common battle ranges...
So in fact, I am missing two things - the rational behind building a super-CA at all, and the idea behind building it as proposed.
I understand Iberia may prefer number of barrels over size of barrel - but on 18kts you can mount 12x24cm plus decent armour.
Just to give an impression of what I think is possible:
CCA42A, Iberian Super Cruiser laid down 1942
Displacement:
17.106 t light; 17.950 t standard; 19.945 t normal; 21.541 t full load
Dimensions: Length overall / water x beam x draught
704,39 ft / 688,98 ft x 72,83 ft x 26,25 ft (normal load)
214,70 m / 210,00 m x 22,20 m x 8,00 m
Armament:
12 - 9,45" / 240 mm guns (4x3 guns), 440,92lbs / 200,00kg shells, 1942 Model
Breech loading guns in turrets (on barbettes)
on centreline ends, evenly spread, 2 raised mounts - superfiring
16 - 4,92" / 125 mm guns (8x2 guns), 63,93lbs / 29,00kg shells, 1942 Model
Dual purpose guns in deck mounts with hoists
on side, evenly spread
16 - 2,24" / 57,0 mm guns (8x2 guns), 5,65lbs / 2,56kg shells, 1942 Model
Automatic rapid fire guns in deck mounts with hoists
on side, evenly spread, all raised mounts
20 - 0,79" / 20,0 mm guns (14 mounts), 0,24lbs / 0,11kg shells, 1942 Model
Anti-aircraft guns in deck mounts
on side, evenly spread, 12 raised mounts
Weight of broadside 6.409 lbs / 2.907 kg
Shells per gun, main battery: 120
Armour:
- Belts: Width (max) Length (avg) Height (avg)
Main: 7,87" / 200 mm 442,91 ft / 135,00 m 13,12 ft / 4,00 m
Ends: Unarmoured
Main Belt covers 99% of normal length
- Gun armour: Face (max) Other gunhouse (avg) Barbette/hoist (max)
Main: 8,66" / 220 mm 4,72" / 120 mm 7,87" / 200 mm
2nd: 1,38" / 35 mm 0,98" / 25 mm 1,38" / 35 mm
3rd: 0,98" / 25 mm 0,79" / 20 mm 0,98" / 25 mm
4th: 0,39" / 10 mm - -
- Armour deck: 3,15" / 80 mm, Conning tower: 1,18" / 30 mm
Machinery:
Oil fired boilers, steam turbines,
Geared drive, 4 shafts, 110.000 shp / 82.060 Kw = 32,13 kts
Range 12.000nm at 15,00 kts
Bunker at max displacement = 3.591 tons
Complement:
838 - 1.090
Cost:
£10,465 million / $41,860 million
Distribution of weights at normal displacement:
Armament: 766 tons, 3,8%
Armour: 5.424 tons, 27,2%
- Belts: 1.920 tons, 9,6%
- Torpedo bulkhead: 0 tons, 0,0%
- Armament: 1.401 tons, 7,0%
- Armour Deck: 2.084 tons, 10,4%
- Conning Tower: 19 tons, 0,1%
Machinery: 2.874 tons, 14,4%
Hull, fittings & equipment: 7.943 tons, 39,8%
Fuel, ammunition & stores: 2.838 tons, 14,2%
Miscellaneous weights: 100 tons, 0,5%
Overall survivability and seakeeping ability:
Survivability (Non-critical penetrating hits needed to sink ship):
27.243 lbs / 12.357 Kg = 64,6 x 9,4 " / 240 mm shells or 2,7 torpedoes
Stability (Unstable if below 1.00): 1,04
Metacentric height 3,4 ft / 1,0 m
Roll period: 16,7 seconds
Steadiness - As gun platform (Average = 50 %): 54 %
- Recoil effect (Restricted arc if above 1.00): 0,91
Seaboat quality (Average = 1.00): 1,07
Hull form characteristics:
Hull has a flush deck
and transom stern
Block coefficient: 0,530
Length to Beam Ratio: 9,46 : 1
'Natural speed' for length: 29,98 kts
Power going to wave formation at top speed: 55 %
Trim (Max stability = 0, Max steadiness = 100): 50
Bow angle (Positive = bow angles forward): 25,00 degrees
Stern overhang: 1,64 ft / 0,50 m
Freeboard (% = measuring location as a percentage of overall length):
- Stem: 29,53 ft / 9,00 m
- Forecastle (35%): 21,00 ft / 6,40 m
- Mid (50%): 21,00 ft / 6,40 m
- Quarterdeck (15%): 21,00 ft / 6,40 m
- Stern: 21,00 ft / 6,40 m
- Average freeboard: 22,19 ft / 6,76 m
Ship space, strength and comments:
Space - Hull below water (magazines/engines, low = better): 77,2%
- Above water (accommodation/working, high = better): 170,7%
Waterplane Area: 35.769 Square feet or 3.323 Square metres
Displacement factor (Displacement / loading): 115%
Structure weight / hull surface area: 144 lbs/sq ft or 702 Kg/sq metre
Hull strength (Relative):
- Cross-sectional: 0,96
- Longitudinal: 1,38
- Overall: 1,00
Hull space for machinery, storage, compartmentation is excellent
Room for accommodation and workspaces is excellent